1. Field of the Invention
The present invention relates generally to well production equipment, and in particular, to flexible production tubing in which a production conduit, power conductors, signal conductors and hydraulic/pneumatic conduits are integrated within a high tensile strength, flexible core.
2. Description of the Prior Art
Various types of artificial lifts are used to bring liquids to the surface of the earth when the pressure of the liquid-bearing reservoir is insufficient to produce the liquids by natural means. The pumping motion of the artificial lift may originate at the surface of the ground, or below as a result of the application of electrical or fluid power to a subsurface pump. In the subsurface-powered rig, a vertically reciprocating pump element at the bottom of the well is actuated by the application of electrical or pneumatic/hydraulic power to a subsurface pump. In the surface-powered rig, a vertically reciprocating pump element at the bottom of the well is actuated by a walking beam pivotally mounted on a Sampson post and connected at one end to the sucker rod string and at the other end to prime mover which supplies power through a Pittman gear for producing the reciprocating motion of the sucker rod string.
Generally, the prime mover consists of an internal combustion engine or electric motor. The cost of this prime mover, as well as its operation and maintenance, is, in many instances, a significant economic factor in the production of liquids from subterranean liquid-bearing reservoirs. The sucker rods are characterized by a short, fast stroke, resulting in low pump efficiency, high power consumption and low recovery rates. Additionally, the steel production rod tubing is subject to failure and must be repaired or replaced at substantial expense from time to time.
An additional limitation of surface-powered rigs which operate a sucker rod through rigid production tubing is that, for practical purposes, the weight of the sucker rod for wells having a producing formation at about ten thousand feet or deeper is excessive, resulting in stretching and early failure of the rods. Moreover, the sucker rod assembly is subject to severe wear in slant-hole or crooked-hole wells. In slant-hole wells, of the type typically drilled offshore, the sucker rod is subjected to severe frictional wear because of the slant of the hole, and is therefore subject to early failure and requires frequent replacement. Frictional wear is also a serious problem in crooked holes in which the well bore follows a helical path. Because of the difficulty of drilling a substantially vertical well at other than shallow depth, effective use of the sucker rod assembly is substantially limited to shallow wells.
For the foregoing reasons, there has been considerable interest in improving pumping systems in which the motive force is provided by electrical or pneumatic/hydraulic power applied to a subsurface pump, thereby eliminating the sucker rods and affording precise control of the pumping action.
Submersible pumps of the type in which the pumping force is derived from electrical, hydraulic, or pneumatic power applied downhole represent a significant improvement over the reciprocating sucker rod approach. According to conventional practice, a submersible pump which is driven down hole by electrical, hydraulic or pneumatic driving means is supported at the end of a long string of steel tubing joints which are coupled to each other by pin and box connections. Power conductors, signal conductors and hydraulic/pneumatic conduits coupled to the downhole pump are bundled together within an external umbilical cable assembly and are secured to the rigid production tubing joints.
Although large diameter steel production tubing has great strength and durability, it requires a substantial capital investment. The procedure of running rigid sections of production tubing into and out of the well bore is complicated by the external umbilical cable assembly. Moreover, the steel production tubing, as well as the pump, is subject to corrosion so that it may be necessary to pull the production tubing from the casing and repair the pump or replace damaged production tubing sections from time to time. When such workover operations become necessary, a portable installation called a workover rig must be brought to the well site and set up. Generally, these rigs consist of a heavy derrick or mast which support block and tackle arrangements that are operable to pull the pipe string from the well. The workover rigs are usually heavy and difficult to erect and must be capable of lifting the substantial load imposed by the rigid pipe string.
An overriding concern in the operation of a producing well is to get the necessary equipment into and out of the well as rapidly and safely as is economically possible. The efficiency of the pipe-handling operation is limited by such factors as the running speed of the hoist rig, the time required to make up or break tool joints during stabbing operations, the time required to mechanically couple and decouple the hoist rig and the pipe string, and the time required to transport lengths of pipe from the pipe string to a storage station during recovery operations and to transport lengths of pipe from the storage station to the pipe string during launching operations. As the length of the pipe string increases to reach the producing formation of deep wells, the pipe handling equipment must safely support the large load of the pipe string and permit the efficient execution of launching and recovery operations while preserving the structural integrity of the pipe string during the handling operations.
It will be appreciated that the substantial capital expenditure required for large diameter steel production tubing, and the expense associated with deployment, recovery, and repair or replacement of the rigid production tubing make its use prohibitive in low production wells, and account for a substantial percentage of the overall production costs of other wells.